Planets and satellites can undergo physical librations, which consist of forced periodic variations in their rotation rate induced by gravitational interactions with nearby bodies. This mechanical forcing may drive turbulence in interior fluid layers such as subsurface oceans and metallic liquid cores through a libration‐driven elliptical instability (LDEI) that refers to the resonance of two inertial modes with the libration‐induced base flow. LDEI has been studied in the case of a full ellipsoid. Here we address for the first time the question of the persistence of LDEI in the more geophysically relevant ellipsoidal shell geometries. In the experimental setup, an ellipsoidal container with spherical inner cores of different sizes is filled with water. Direct side view flow visualizations are made in the librating frame using Kalliroscope particles. A Fourier analysis of the light intensity fluctuations extracted from recorded movies shows that the presence of an inner core leads to spatial heterogeneities but does not prevent LDEI. Particle image velocimetry and direct numerical simulations are performed on selected cases to confirm our results. Additionally, our survey at a fixed forcing frequency and variable rotation period (i.e., variable Ekman number, E) shows that the libration amplitude at the instability threshold varies as ∼E0.65. This scaling is explained by a competition between surface and bulk dissipation. When extrapolating to planetary interior conditions, this leads to the E1/2 scaling commonly considered. We argue that Enceladus' subsurface ocean and the core of the exoplanet 55 CnC e should both be unstable to LDEI.
Jupiter’s dynamics shapes its cloud patterns but remains largely
unknown below this natural observational barrier. Unraveling the underlying
three-dimensional flows is thus a primary goal for NASA’s ongoing Juno
mission that was launched in 2011. Here, we address the dynamics of large Jovian
vortices using laboratory experiments complemented by theoretical and numerical
analyses. We determine the generic force balance responsible for their
three-dimensional pancake-like shape. From this, we define scaling laws for
their horizontal and vertical aspect ratios as a function of the ambient
rotation, stratification and zonal wind velocity. For the Great Red Spot in
particular, our predicted horizontal dimensions agree well with measurements at
the cloud level since the Voyager mission in 1979. We additionally predict the
Great Red Spot’s thickness, inaccessible to direct observation: it has
surprisingly remained constant despite the observed horizontal shrinking. Our
results now await comparison with upcoming Juno observations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.